MPI_BUFFER_ATTACH
provides to MPI a buffer in the application's
memory to be used for buffering outgoing
messages. The buffer is used only by messages sent in buffered mode.
Only one buffer can be attached at a time (per process).

MPI_BUFFER_DETACH detaches the buffer currently associated
with MPI. The call returns the
address and the size of the detached buffer. This operation
will block until all messages currently in the buffer have been transmitted.
Upon return of
this function, the user may reuse or deallocate the space taken by the buffer.

Now the question arises: how is the attached buffer to be
used? The answer is that MPI must behave
as if
buffer policy
outgoing message
data were buffered by the sending process, in the specified buffer space,
using a circular, contiguous-space allocation policy.
We outline below a model implementation that defines this policy.
MPI may provide more buffering, and may use a better buffer allocation
algorithm
than described below. On the other hand, MPI may signal an error
whenever the
simple buffering allocator described below would run out of space.